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WASHINGTON — Striking back, the Obama administration yesterday branded the leak of more than a quarter-million sensitive files an attack on the United States and raised the prospect of criminal prosecution against the online site WikiLeaks.

Secretary of State Hillary Rodham Clinton asserted that WikiLeaks acted illegally in posting the material. She said the administration was taking “aggressive steps to hold responsible those who stole this information.’’

Clinton spoke in between calls to foreign capitals to make amends for scathing and gossipy memos never meant for foreign eyes.

“This disclosure is not just an attack on America’s foreign policy interests,’’ she said. “It is an attack on the international community, the alliances and partnerships, the conversations and negotiations that safeguard global security and advance economic prosperity. I am confident that the partnerships that the Obama administration has worked so hard to build will withstand this challenge.’’

The government is mounting a criminal investigation, Attorney General Eric Holder said, adding that anyone found to have broken American law will be held responsible.

Holder said the latest disclosure, involving classified and sensitive State Department documents, jeopardized the security of the nation, its diplomats, intelligence assets, and relationships with foreign governments.

The Pentagon yesterday detailed new security safeguards, including restraints on small computer flash drives, to make it harder for anyone to copy and reveal so many secrets.

The young Army private suspected of stealing the diplomatic memos, many of them classified, and feeding them to WikiLeaks may have defeated Pentagon security systems using little more than a Lady Gaga CD and a portable computer memory stick.

The soldier, Bradley Manning, has not been charged in the latest release of internal US government documents. But officials said he is the prime suspect partly because of his own description of how he pulled off a staggering theft of classified and restricted material.

“No one suspected a thing,’’ Manning told a confidant afterward, according to a log of his computer chat published by Wired.com. “I didn’t even have to hide anything.’’

Manning is charged in military court with taking other classified material later published by the online clearinghouse WikiLeaks.

It is not clear whether others, such as WikiLeaks executives, might be charged separately in civilian courts.

Colonel Dave Lapan, a Pentagon spokesman, said it was possible that many people could be held accountable if they were found to have ignored security protocols or somehow enabled the download without authorization.

In his Internet chat, Manning described the conditions as lax to the point that he could bring a homemade music CD to work with him, erase the music, and replace it with secrets.

Wired.com published a partial log of Manning’s discussions with hacker R. Adrian Lamo in June.

By his own admission, Manning was apparently able to pull material from outside the Pentagon, including documents he had little obvious reason to see.

He was arrested shortly after those chats last spring and is awaiting trial on the earlier charges. Manning’s civilian lawyer, David E. Combs, declined to comment.

Lapan said the WikiLeaks experience has encouraged discussion within the military about how better to strike a balance between sharing information with those who need it and protecting it from disclosure.

Pentagon spokesman Bryan Whitman said the WikiLeaks case revealed vulnerable seams in the information-sharing systems used by multiple government agencies.

Some of those joint systems were designed to answer another problem: the failure of government agencies to share what they knew before the Sept. 11, 2001, attacks.